Currently, packet transport systems are being used to distribute entertainment programming. For example, a satellite packet transport system may transport a plurality of television channels to viewer locations, each television channel containing a video signal and one or more audio signals. In addition, it has been proposed to include a program data signal with each television channel to permit a processor located in the television receiver to execute a program which can interact with the viewer. Such a satellite packet transport system will transmit a plurality of different component signals (e.g. video, audio and program data).
It has also been proposed to provide a low data rate service for slower throughput data transport. For example, a component signal provider may wish to transfer data having a predetermined size, such as a data file for delivery over a predetermined, relatively long, time period. For example, a bank may wish to transmit a block of transactions from one location to another overnight.
In U.S. patent application Ser. No. 08/442,429, entitled A METHOD AND APPARATUS FOR OPERATING A TRANSPORT STREAM ENCODER TO PRODUCE A STREAM OF PACKETS CARRYING DATA REPRESENTING A PLURALITY OF COMPONENT SIGNALS, and filed May 16, 1995, now U.S. Pat. No. 5,574,505 incorporated by reference, a transport packet stream encoder is disclosed which produces a composite packet stream containing a plurality of component signals. The composite packet stream is partitioned into successive groups of packet slots. Each packet slot in a group has associated with it a priority ordered list of acceptable component signals for that packet slot. The data from a selected one of these component signals may be placed in that packet slot. When a packet is to be formed, the priority list associated with that packet slot is traversed in priority order, and the first component signal source which has enough data waiting to be transmitted to fill a packet is assigned to fill that packet slot. If none of the component signal sources in the priority list have enough data, then a null packet is produced in that packet slot. The priority lists are generated in such a manner as to provide the level of throughput which the providers of the component signals have purchased from the transport provider.
As stated in that application, this arrangement gives the flexibility to assign a high priority to low data rate components, and a low priority to high data rate components in a packet slot. This permits low data rate signals sufficient access to packet slots to maintain their required throughput, while allowing those slots to be filled by high data rate component data if the low data rate component does not have sufficient data available to fill a packet, and thus avoid the generation of a wasteful null packet.
However, in such a packet transport system carrying many component signals, the throughput required by some of the component signals varies dynamically. U.S. patent application Ser. No. 08/576,527, entitled OPTIMIZING PERFORMANCE IN A PACKET SLOT PRIORITY PACKET TRANSPORT SYSTEM, and filed Dec. 21, 1995, now U.S. Pat. No. 5,729,292 incorporated by reference, discloses a transport stream encoder which may be dynamically optimized to the current required throughput of the component signals carried by the packet stream. The packet transport system disclosed in this application is similar to that disclosed in 08/442.429 except that the contents of the plurality of priority lists may be dynamically updated based on some predetermined parameter.
For example, such a transport stream encoder can adapt its operation to changing throughput requirements of the various component signals due to scheduled changes in programming, and thus optimize its operation, e.g. providing the throughput required by each component while minimizing the number of null packets. A clock may provide a signal to such a system to indicate when the programming has changed (such as at the hour and half-hour). When the clock signal is received, the priority lists are updated to optimally configure the packet transport system for the new mix of component signal sources, and their content.
Alternatively, the priority lists may be updated based on the content of the packet stream. In such a transport stream encoder, each packet produced by the packet stream encoder is analyzed by a data collector to determine the identity of the component signal source whose data is contained in that packet. The entries in the plurality of lists are updated in response to the gathered data, for example, to take advantage of a lull in the data rate of one component signal. In this case, the entries in the plurality of lists are updated to decrease the priority of the component signal experiencing a lull, thereby increasing the priorities of the other component signals. In this manner, the operation of the transport stream encoder may be optimized to the instantaneous changes in the data rate of the component signal sources.
Referring again to the example of a bank desiring to transmit a file of transactions from one location to another overnight, the packet transport stream encoder of either 08/442,429 or 08/576,524 may be used to perform this transfer. By placing an entry corresponding to the bank data component signal source at the bottom of each priority list, when no other component signal has enough data to form a packet in that packet slot, a packet containing some of the bank data will be formed and transmitted, instead of a null packet. Because a null packet will never be transmitted so long as there is bank data to transmit, all generated packets will carry data. The transfer of information in this manner is called an opportunistic transfer. But, in the above system, there can be no guarantee that the bank data will be transmitted within a specified time period. It is desirable that a packet transport system be able to provide guaranteed delivery of opportunistic data within a specified time period.